Kelly K. Bradbury

Mineralogic and textural analyses of drill cuttings from the San Andreas Fault Observatory at Depth (SAFOD) boreholes: Initial interpretations of fault zone composition and constraints on geologic models

We examine drill cuttings from the San Andreas Fault Observatory at Depth (SAFOD) boreholes to determine the lithol- ogy and deformational textures in the fault zones and host rocks. Cutting samples repre- sent the lithologies from 1.7-km map distance and 3.2-km vertical depth adjacent to the San Andreas Fault. We analyzed two hun- dred and sixty-six grain-mount thin-sections at an average of 30-m-cuttings sample spac- ing from the vertical 2.2-km-deep Pilot Hole and the 3.99-km-long Main Hole. We iden- tify Quaternary and Tertiary(?) sedimen- tary rocks in the upper 700 m of the holes; granitic rocks from 760-1920 m measured depth; arkosic and lithic arenites, interbed- ded with siltstone sequences, from 1920 to ~3150 m measured depth; and interbed- ded siltstones, mudstones, and shales from 3150 m to 3987 m measured depth. We also infer the presence of at least fi ve fault zones, which include regions of damage zone and fault core on the basis of percent of cata- clasite abundances, presence of deformed grains, and presence of alteration phases at 1050, 1600-2000, 2200-2500, 2700-3000, 3050-3350, and 3500 m measured depth in the Main Hole. These zones are correlated with borehole geophysical signatures that are consistent with the presence of faults. If the deeper zones of cataclasite and alteration intensity connect to the surface trace of the San Andreas Fault, then this fault zone dips 80-85° southwest, and consists of multiple slip surfaces in a damage zone ~250-300 m thick. This interpretation is supported by borehole geophysical studies, which show this area is a region of low seismic velocities, reduced resistivity, and variable porosity.

Composition, Alteration, and Texture of Fault-Related Rocks from Safod Core and Surface Outcrop Analogs: Evidence for Deformation Processes and Fluid-Rock Interactions

We examine the fine-scale variations in mineralogical composition, geochemical alteration, and texture of the fault-related rocks from the Phase 3 whole-rock core sampled between 3,187.4 and 3,301.4 m measured depth within the San Andreas Fault Observatory at Depth (SAFOD) borehole near Parkfield, California. This work provides insight into the physical and chemical properties, structural architecture, and fluid-rock interactions associated with the actively deforming traces of the San Andreas Fault zone at depth. Exhumed outcrops within the SAF system comprised of serpentinite-bearing protolith are examined for comparison at San Simeon, Goat Rock State Park, and Nelson Creek, California. In the Phase 3 SAFOD drillcore samples, the fault-related rocks consist of multiple juxtaposed lenses of sheared, foliated siltstone and shale with block-in-matrix fabric, black cataclasite to ultracataclasite, and sheared serpentinite-bearing, finely foliated fault gouge. Meters-wide zones of sheared rock and fault gouge correlate to the sites of active borehole casing deformation and are characterized by scaly clay fabric with multiple discrete slip surfaces or anastomosing shear zones that surround conglobulated or rounded clasts of compacted clay and/or serpentinite. The fine gouge matrix is composed of Mg-rich clays and serpentine minerals (saponite ± palygorskite, and lizardite ± chrysotile). Whole-rock geochemistry data show increases in Fe-, Mg-, Ni-, and Cr-oxides and hydroxides, Fe-sulfides, and C-rich material, with a total organic content of >1 % locally in the fault-related rocks. The faults sampled in the field are composed of meters-thick zones of cohesive to non-cohesive, serpentinite-bearing foliated clay gouge and black fine-grained fault rock derived from sheared Franciscan Formation or serpentinized Coast Range Ophiolite. X-ray diffraction of outcrop samples shows that the foliated clay gouge is composed primarily of saponite and serpentinite, with localized increases in Ni- and Cr-oxides and C-rich material over several meters. Mesoscopic and microscopic textures and deformation mechanisms interpreted from the outcrop sites are remarkably similar to those observed in the SAFOD core. Micro-scale to meso-scale fabrics observed in the SAFOD core exhibit textural characteristics that are common in deformed serpentinites and are often attributed to aseismic deformation with episodic seismic slip. The mineralogy and whole-rock geochemistry results indicate that the fault zone experienced transient fluid–rock interactions with fluids of varying chemical composition, including evidence for highly reducing, hydrocarbon-bearing fluids.

Hydrogeologic heterogeneity of faulted and fractured Glass Mountain bedded tuffaceous sediments and ash-fall deposits: The Crucifix site near Bishop, California

Lithologic, macrostructural, microstructural, geophysical, and in situ gas permeability data from a natural exposure of highly porous, faulted and fractured tuffaceous sediments and interbedded ash-fall deposits near Bishop, California, are presented and analyzed in relation to published geologic information. This natural analog study was motivated by the need to evaluate potential length scales over which lateral flow diversion might occur above and within the nonwelded Paintbrush Tuff at Yucca Mountain, Nevada. Lateral diversion of flow in the overlying Paintbrush Tuff was previously proposed by others as a natural barrier that might protect a proposed high-level radioactive waste and spent nuclear fuel repository from percolating water. Because the length scale for capillary barrier breakthrough and leakage is decreased in the presence of subvertical structural heterogeneities, we characterized a horst-bounding fault, small-displacement normal faults within a footwall deformation zone, and secondary heterogeneities within two beds dissected by the faults. Critical deformation-related features that may influence fluid flow within bedded tuffaceous sediments include (1) permeability anisotropy imposed by steeply dipping faults and stratigraphic layering; (2) fault zone widths and styles, which are dependent on bed thickness and ash, glass, and clay content; and (3) fracture intensities and overprinting mechanisms (associated with fault deformation and vertical and nonvertical fracture orientations), which strongly influence the hydrogeologic heterogeneity of units they dissect. Microstructural analysis reveals structurally induced porosity variations at the micrometer to millimeter scale, gas permeability data show the influence of deformation on permeability at the centimeter to tens of centimeters scale, and resistivity and ground-penetrating radar data show lateral variations on the meter to tens of meters scale in horizontally bedded layers. All together, these observations and data show heterogeneity over seven orders of magnitude of length scale. Structurally enhanced porosity and permeability heterogeneities will tend to limit the length scale of lateral flow diversion, redirect flow downward, and enhance vertical fluid movement within the vadose zone.

Structure and Hydrogeology of Deformed Sedimentary Bedrock Aquifers, Western Summit County, Utah

The Snyderville basin, near Park City in western Summit County, Utah, has experienced significant water shortages coupled with a 50 percent growth rate in the past 10 to 15 years. Recent development rests directly on complexly folded and fractured sedimentary bedrock aquifers in the hanging wall of the Mount Raymond thrust. Detailed geologic and fracture scanline mapping coupled with structural analyses in the Pinebrook subdivision, one site within the Snyderville basin demonstrating abrupt hydrogeologic changes, provide a clearer picture of the local hydrogeologic setting. The dominant map-scale structure is the Twomile Canyon anticline. Several macroscopic faults cut this fold, including the Toll Canyon fault, a backthrust off the Mount Raymond thrust. Fracture orientations and densities vary within meters across the Twomile Canyon anticline as a function of lithology and position relative to macroscopic faults. Exposures of the Toll Canyon fault show that the width and lithologic composition of the fault core and related damaged zone are a function of lithology, and the fault strongly controls fracture permeability. Damage zones in limestones and sandstones with high fracture intensities may be regions of enhanced permeability, whereas shale smears and clay gouge adjacent to the fault core act as barriers to fluid flow. A conceptual model of the subsurface in the Pinebrook study area has been developed, and several test well sites have been proposed based on this model and field observations. The target formation, structural position, fracture intensity, local hydrogeology, and accessibility were factors considered in locating these wells.

Geology and in situ stress of the MH-2 borehole, Idaho, USA: Insights into western Snake River Plain structure from geothermal exploration drilling

Project HOTSPOT, the Snake River Scientific Drilling Project (International Continental Scientific Drilling Program), tested for deep geothermal resources and examined the petrology of volcanic rocks with three drillholes in the central and western Snake River Plain (western USA). The MH-2 drillhole targeted fractured crystalline and hydrothermally altered basalt in the area of the Mountain Home Air Force Base (Idaho) to a total depth of 1821 m. At 1745 m depth the drillhole encountered flowing artesian hydrothermal fluids of at least 150 °C. We integrate geological analyses of core, image log, and borehole geophysical data, and in situ stress analyses to describe the structural environment that produces permeability for artesian flow. The rocks in the lower 540 m of the drillhole consist of basalt flows as much as 30 m thick, altered basalt, and thin sedimentary horizons. The mechanical stratigraphy is defined by nine mechanical horizons that are in three ranges of rock strength on the basis of experimentally determined strength data, core logging, and geophysical log signatures. Hydrothermal alteration products and mineralization in the core are associated with three highly faulted sections; the lowermost section is associated with the zone of flowing thermal water. Shear slip indicators on faults observed in core indicate slip ranging from pure strike slip to normal failure mechanisms in the stronger horizons. The borehole breakouts indicate that the maximum horizontal stress, SH, is oriented 047° ± 7°, and drilling-induced tensile fractures indicate that SH is oriented at 67° ± 21°. The in situ stress orientations exhibit little variation over the depth of the measured interval, but the SH magnitude varies with depth, and is best explained by an oblique normal fault stress regime. The geomechanical model indicates that if pore pressures at depth are elevated above the normal hydrostatic gradient, as observed here, the system has the potential to deform by mixed normal and strike-slip failure. Our observations and interpretations suggest that the MH-2 borehole was drilled into oblique normal faults that intersect a buried 300°-trending fault block masked by the basaltic volcanic complex. These data indicate that the transition from the central to western Snake River Plain is characterized by complex structures developed in response to a transitional stress state related to Snake River Plain and western Basin and Range stress regimes. The western Basin and Range stress and tectonic regime may extend from northern Nevada into western Idaho and may enhance the potential for geothermal resources by creating interconnected fracture and fault-related permeability at depth.

CARBONACEOUS FAULT-RELATED ROCKS IN SAFOD PHASE III CORE: INDICATORS OF FLUID-ROCK INTERACTION AND STRUCTURAL DIAGENESIS DURING SLIP

• At elevated temperatures, many carbon-rich fault zones are subjected to thermal maturation, fluid-rock interactions, and/or shear-induced phase transformations, often yielding various fault weakening agents. • Black carbonaceous material documented by Bradbury et al. (2011; 2015) in SAFOD Phase III core exhibits intense comminution, shear-induced slip localization, fragmented shear zones, stylolites, calcite-cemented breccia, calcite veins of varying trace element chemistry, and calcite vein fragments in the wall rock. • Presence of calcite-cemented carbonaceous ultracataclasites in creeping segments of the Central Deforming Zone (CDZ) and Southwest Deforming Zone (SDZ) of the San Andreas Fault raises questions regarding the source of the carbonaceous material, and the nature of thermochemical reactions and fluid-rock interactions that promote dynamic weakening and strength recovery in carbon-rich fault gouges during the seismic cycle. Overview